If you've been reading recent media coverage on Aids vaccine research, such as the article in the Independent on April 24, you might think developing an Aids vaccine is a mission doomed to fail. If one setback always provoked that much pessimism about scientific efforts, developing new medicines and vaccines to address the world's ills would be an impossibly frustrating business. Fortunately, researchers are not so easily knocked off course. They know that breakthroughs are a product of patience and perseverance.

The latest media interest in Aids vaccine research and development follows on recent discussions within the science community on how that work should continue in the wake of disappointing results in a clinical trial that tested a vaccine candidate made by Merck & Co. No serious scientists have suggested giving up on the effort to develop an Aids vaccine. While it is impossible to predict with certainty a successful outcome, there is strong scientific evidence indicating that HIV can be controlled and infection prevented by activating the right immune response with the right vaccine.

Today, on the 10th World Aids Vaccine Day, 25 years since the discovery of HIV, why do we not yet have an Aids vaccine? I think many forget that Aids vaccine work is a relatively young and pioneering science. Following disappointing results in the early 1980s, the effort withered. Serious investment and access to resources, including scientific expertise and research infrastructure in the countries most affected by Aids, has only been available since the late 1990s.

What's more, scientists must explore new ways of designing vaccines for HIV because conventional methods are not appropriate. For instance, the "live-attenuated" or "whole-killed" virus approach, which uses a weakened or killed virus that cannot cause infection, is very effective in vaccines such as those against polio, measles and influenza. But this approach is not considered safe for HIV, out of concern that the crippled virus would not be adequately inactivated or could revert to its disease-causing form.

But that it not to say that Aids vaccine researchers have not already made advances. We have learned a great deal about HIV - arguably more than about any other pathogen - and about its interaction with our immune system. Only in recent years have scientists from around the world begun to collaborate to turn this knowledge into novel designs for HIV vaccine candidates. Importantly, developing countries are playing a crucial role in this effort, following significant investment there in the infrastructure, scientists and technicians necessary to conduct clinical research.

With 33 million people living with HIV around the world, and 2.5 million newly infected last year, it is clear that existing interventions are not successfully controlling this pandemic, especially not in developing countries, which account for 95% of new infections. Investing in Aids vaccine research is investing in an opportunity not just to ameliorate the suffering caused by Aids but to actually end the epidemic. Certainly, that investment should not be at the expense of scaling up access to proven prevention strategies or of delivering much needed medication to people infected with HIV. Still, we cannot afford to neglect the vaccine effort. Vaccines remain the most powerful tool we have to control infectious diseases.

Almost certainly there is a long way to go before we will have an effective Aids vaccine, but that is not news in vaccine development. It took decades for other vaccines, such as those against smallpox and polio, to be developed. Today, no one questions whether it was worth investing in these vaccines; most of us have been vaccinated and we live free from fear of infection by many deadly bacteria and viruses. Given the commitment of those who are in this field, I expect the same will be said one day of HIV.